A method based on Fabry-Perot (F-P) interferometer is proposed to detect the displacement of the micro-cantilever of atomic force microscopy. A hemispherical F-P cavity composed of a spherical mirror and a micro-cantilever is designed. A single frequency laser is used as the light source. The laser beam is reflected multiple times in the hemispherical F-P cavity and returned to the original path to form an interference. According to the principle of multi-beam interference, the relationship between the interference intensity and the cavity length has been determined and the deflection of the micro-cantilever has been calculated by using the intensity demodulation method. Finally, a micro-cantilever displacement detection system has been built. The experiment obtained a linear fit of 99.9% in the working range of 30 nm, and the minimum displacement resolution of the micro-cantilever was 0.26 nm. The experimental results are consistent with the theoretical analysis, which proves the practicability of the system.